1. Field of the Invention
The present invention relates to an electromagnetic induction-type absolute position transducer in which variation of the signal strength depending on positions on a scale is small. More particularly, the present invention relates to an electromagnetic induction-type absolute position transducer comprising a scale with a plurality of scale loops functioning as coils, the scale loop having a set of loop portions including at least two loop portions, which are arranged along a measuring axis at different wavelength intervals to constitute a plurality of tracks arranged along the measuring axis at different wavelengths, and a connecting pattern portion connecting the corresponding two loop portions with each other. The electromagnetic induction-type absolute position transducer is suitably used in an electronic caliper, an electronic micrometer, an indicator, a linear scale, a linear gauge, and the like.
2. Description of the Related Art
The assignee of the invention has proposed an electromagnetic induction-type absolute position transducer having N (Nxe2x89xa72) tracks of different wavelengths, in Japanese Patent Unexamined Publication No. 2000-180209 (filed based on U.S. patent application Ser. No. 09/213,268). As shown in FIG. 10, the proposed transducer has a scale 10 and a readhead 20 which are relatively movable along a measuring axis X. The readhead 20 includes at least one magnetic flux sensor (receiving coils 22 and 24). The scale 10 has a plurality of closed-loop coupling loops (hereinafter, referred to as scale loops) which extend along the measuring axis. The scale loops function as coils. Each of the scale loops includes a first loop portion 12, a second loop portion 14, and a connecting pattern portion 16 connecting the first and second loop portion with each other. The first loop portions 12 are arranged at intervals corresponding to a first wavelength xcex1 along the measuring axis. The second loop portions 14 are arranged at intervals corresponding to a second wavelength xcex2 which is different from the first wavelength xcex1.
In FIG. 10 a driving circuit 30 selectively outputs a time-varying drive signal to either a first transmitting coil 26 or a second transmitting coil 28. An amplifying circuit 32 amplifies signals from the receiving coils 22 and 24. A calculation device 34 A/D-converts an output of the amplifying circuit 32 and calculates an absolute position from the phase difference between the tracks.
In FIG. 10, the first loop portions 12 are arranged on one side of the second loop portions 14. The first loop portions 12 may be alternately arranged on both the sides of the second loop portions 14 as shown in FIG. 11.
FIG. 12 enlargedly shows connecting stats of 0-th and n-th first loop portions 12 and second loop portions 14 in the scale loops of FIG. 10. FIG. 13 shows variation of the signal strength with respect to the length of the scale loop (hereinafter, referred to as scale length). As shown in FIGS. 12 and 13, in the electromagnetic induction-type absolute position transducer of the related art, the connecting pattern portion 16, which connects loop portions (the first loop portion 12 and the second loop portion 14) of each scale loop in the scale 10, is longer as the scale length is larger. Accordingly, if the pattern widths of the scale loops (pattern widths A of the first and second loop portions, and pattern widths B of the connecting patterns 16) are kept to be constant along the measuring axis as shown in FIG. 12, an output signal of the readhead 20 is lowered in strength as indicated by the solid line A in FIG. 14 as the scale length is larger. That is, the output signal of the readhead is lowered in strength as the readhead moves away from the position of the scale loop having the shortest length connecting pattern portion (hereinafter, referred as 0-th scale loop) Here, the scale pattern of the 0-th scale loop is referred as shortest connecting pattern.
When the signal strength is changed depending on the scale position as described above, the setting range of the gap between the scale and the readhead is inevitably narrowed in order to ensure a sufficient signal strength even when the signal strength is reduced. Therefore, the accuracy of components and an assembling process must be enhanced. Further, in the case where, as proposed by the assignee of the invention in U.S. patent application Ser. No. 09/804,300, an erroneous operation due to, for example, the readhead or a breakage of the scale is detected on the basis of the signal strength, the detection sensitivity cannot be set to be high.
The invention has been conducted in order to solve the problems of the related art. It is an object of the invention to provide an electromagnetic induction-type absolute position transducer in which variation of an output signal depending on positions on the scale is small.
To achieved the above-mentioned object of the invention, there is provided with an electromagnetic induction-type absolute position transducer comprising a scale with a plurality of scale loops functioning as coils, the scale loop having a set of loop portions including two loop portions, which are arranged along a measuring axis at different wavelength intervals to constitute a plurality of tracks arranged along the measuring axis at different wavelengths, and a connecting pattern portion connecting corresponding two loop portions with each other. In the electromagnetic induction-type absolute position transducer, at least part of pattern widths constituting the scale loops are changed in accordance with the length of the connecting pattern portion (hereinafter, as referred as connecting pattern length). With such configuration, a change of the signal strength due to a change of the connecting pattern length is prevented from occurring.
In the above-mentioned electromagnetic induction-type absolute position transducer, it is preferable that at least part of the pattern widths may be gradually increased as the connecting pattern length becomes longer with respect to a shortest connecting pattern.
Further, according to the present invention, there is provided a position measuring device which uses the above-mentioned electromagnetic induction-type absolute position transducer.
In the electromagnetic induction-type absolute position transducer and the position measuring device using the same according to the present invention, in the scale pattern where the loop portions are arranged at constant pitches (in the related art, the first loop portions are arranged at the pitch xcex1, and the second loop portions at the pitch xcex2), at least part of the pattern widths constructing the scale loop are changed depending on the connecting pattern length without changing the pitches and the loop center distances of the first and second loop portions. For example, at least part of the pattern widths is stepwisely (gradually) increased as the connecting pattern length becomes longer with respect to the shortest connecting pattern (in this example, the 0-th scale loop: also referred to as a scale coil). According to this configuration, the amount of the attenuation of the signal strength due to increase of the scale length such as that indicated by the solid line A in FIG. 14 can be complemented by increasing the current flowing through the scale loop. Therefore, attenuation of the signal strength depending on increase of the connecting pattern length is prevented from occurring, whereby the strength of the output signal of the readhead can be kept to be constant as indicated by the solid line B in FIG. 14.